A three-year-old male German Shepherd was referred to the Oncology service at the Vetmeduni Vienna due to recurrent and progressive cutaneous round-cell tumours, diagnosed as poorly differentiated mast-cell tumours in histopathological examination of excisional biopsies. The dog was originally presented at the referring veterinarian with the one week history of two skin nodules in the left shoulder and lumbar region. Initial staging prior to mass excision included CBC, a biochemistry panel and abdominal ultrasound. Blood work was unremarkable and abdominal ultrasound depicted a 1.4 cm splenic nodule. Skin tumours recurred four days after resection as disseminated nodules in both scar regions and all over the body. Furthermore, the dog started to show signs of systemic illness with fever, lethargy and vomiting and was therefore hospitalized at the Vetmeduni Vienna at presentation. Re-staging of the patient confirmed not only cutaneous disease progression but also systemic progres-sion with two splenic masses of 4 and 5 cm diameter, three nodules in the left kidney and mild effusion in the chest and abdomen. Neoplastic round cells in the abdominal effusion were identified as lymphoid precursor cells but not as mast cells. Cytological examination of the skin nodules identified the same neoplastic cells as in the effusion. Re-evaluation of histopathology slides, clonality testing and flow cytometry established the final diagnosis of cutaneous non-epitheliotropic cutaneous T-cell lymphoma with systemic involvement. Further disease progression in skin and abdomen was noticed as early as two weeks after the start of a LCHOP chemotherapy protocol and the dog was euthanized due to the guarded prognosis. The case demonstrates that cutaneous lymphoma is a great impostor among cutaneous round cell tumours.